The present invention relates to a beam position monitor used with a circular charged particle accelerator and a method for measuring the position of a beam of charged particles in such an accelerator.
FIG. 13 shows a typical example of a known circular charged particle accelerator in the form of a synchrotron. In general, a beam of charged particles injected into the synchrotron is curved by a bending electromagnet 1 to circulate in a hollow ring or annular duct. The continuous beam circulating in the ribs is bunched and accelerated by high-frequency electromagnetic fields, i.e., charged particles in the beam are congregated or clustered and accelerated. The whole charged particles of the bunched beam circulate along a single beam orbit while oscillating therearound. A maximum magnitude of such oscillations is called a beam size.
Although the beam orbit is designed to pass through the center of a tubular vacuum chamber in the circular ring, the actual beam orbit can be deviated from the center of the vacuum chamber due to, for example, variations in the installation position of the bending electromagnet 1 and the like. Since the beam should be present in the vacuum chamber, such a deviation of the beam orbit reduces an area or space in which the beam can perform an orbital movement, thus resulting in an accordingly decreased beam current which can be introduced and accelerated. To avoid this, a multitude of beam position monitors 2 are disposed around the ring for detecting or measuring the position of the beam in the vacuum chamber therein and generating a corresponding signal, based on which unillustrated correction electromagnets disposed around the ring operate to properly correct or modify the position of the beam orbit.
FIG. 14 shows in cross section an example of a conventional beam position monitor 2 as published at the Sixth Symposium on Accelerator Science and Technology concerning the accelerator science and technology held in Tokyo in the year of 1987. In this Figure, the monitor 2 includes a hollow vacuum chamber 3 through which a beam 5 of charged particles passes, and a plurality of (i.e., four in the illustrated example) button electrodes 4A through 4D disposed in the vacuum chamber 3.
Now, the operational principle of the beam position monitor 2 of FIG. 14 will be described in brief. Each time the beam 5 of charged particles passes through an illustrated specific position in the vacuum chamber 3, charges are induced in the respective electrodes 4A to 4D. At this time, the largest amount of charge is induced in the electrode 4A which is the nearest to the beam 5, whereas the least amount of charge is induced in the electrode 4C which is the remotest from the beam 5. When the beam 5 passes through the center of the vacuum chamber 3, the amounts of charge induced in the respective electrodes 4A to 4D become equal to each other. Though not illustrated, the electrodes 4A to 4D are connected to ground through resistors, respectively, so that a voltage across each resistor is measured to sense the position of the beam. In this regard, it is to be noted that the magnitude of the voltage thus induced is proportional to the amount of charge of the beam differentiated by time, so a continuous beam results in zero voltage induced. Therefore, the beam must be bunched.
Accordingly, the beam position monitor generates an output signal in the form of a series of pulses, which, if expanded so a Fourier series, becomes equal to a multiple of the frequency of the orbital movement of the beam. As shown in FIG. 15, a signal processing circuit 6 processes an output signal of the beam position monitor in the following manner in order to perform a signal amplification at a high S/N ratio. Namely, it selects a certain high-frequency component among the monitor output signal, amplifies is in a heterodyne manner, and then converts in from analog into digital form which is finally read out by a computer 7.
A conventional process of correcting the beam position by use of the beam position monitor 2 of FIG. 14 will be described below while referring to FIG. 16. In this Figure, the beam position monitor 2 is similar to that of FIG. 14 although the shape of the vacuum chamber 3 is somewhat different from that of FIG. 14.
First, as shown in FIG. 16, a drive table 9 is moved in an x-axis or y-axis directions so as to accordingly displace the beam position monitor 2 mounted thereon while an antenna 8 in the form of a fine wire inserted in the cylindrical hollow interior of the vacuum chamber 3 generates high-frequency pulses. During the movement of the beam position monitor 2, voltages across the respective electrodes 4A to 4D are measured to calculate the following two voltage ratios: EQU H=(A+D-C-B)/(A+B+C+D) EQU V=(A+B-C-D)/(A+B+C+D)
where A is a voltage across the electrode 4A; B is a voltage across the electrode 4B; C is a voltage across the electrode 4C; and D is a voltage across the electrode 4d. FIG. 17 shows these voltage ratios H, V when the antenna 8 is being moved relative to the beam position monitor 2 in the x-axis or y-axis direction. The ratios M, V are plotted on the H-V plane when the antenna 8 is moving in the y-axis direction with the value of x being held constant (e.g., x=0, .+-.1 mm, .+-.2 mm, . . . ), or in the x-axis direction with the value of y being held constant (e.g., y=0, .+-.1, .+-.2, . . . ).
Thus, by plotting the voltage ratios H, V output from the beam position monitor 2 on the H-V plane as shown in FIG. 17 while changing the number of horizontal or vertical solid lines per unit length, the position of the antenna 8 can be detected and then corrected to a proper location based thereon. For this reason, these solid lines are called "correction curves". In case of the beam position monitor 2, the position of the beam can be recognized based on the voltage ratios H, V output from the monitor 2 and the correction lines plotted on the H-V plane. The apparent asymmetry of the correction lines with respect to the center of the H-V plane is due to variations in the mounting of the electrodes 4A to 4D.
According to the conventional correction method of the beam position monitor 2 as described above, accurate position measurements can be performed in cases where the size or diameter of the beam is as small as that of the antenna 8, but it becomes impossible even if the beam center is concentric with the antenna 8, as long as the beam diameter or size is larger than the antenna diameter with the beam center being deviated from center of the beam-position monitor 2. In a synchrotron for electromagnetically accelerating a low energy beam injected therein, it is very difficult to accurately detect the position of the beam since the beam diameter at the time of injection thereof is very large.